Olefin polymers have been used for a number of years to produce film material by an extrusion process commonly referred to as the "bubble" technique. The bubble technique involves melting polymer pellets or granules, extruding the melt from an annular die as a film in the form of a bubble and withdrawing the film from the die at a rate greater than the rate of extrusion while maintaining positive air pressure within the interior of the bubble. Film, so produced, is oriented or stretched in the machine or longitudinal direction and in the transverse direction and as a result is characterized, generally, by improved mechanical properties.
As a general rule, there are so-called "dead spots " or "hang-up" areas in the extruder wherein molten polymer tends to stagnate. Consequently, portions of the polymer melt remain in the extruder for extended periods of time and undergo premature crosslinking or thermal degradation. In time, as these crosslinked or thermally degraded portions of polymer are flushed out into the main polymer stream, they cause optical degradation, known as pin-striping and gel-streaking, in the extruded film. Pin-striping is manifested by haze bands which are aligned in the machine direction of the extruder. Gel-streaking is manifested by surface roughness.
Optical degradation is accentuated in film extruded from substantially linear, low density ethylene polymers prepared under conditions of low pressures in the presence of transition metal catalysts known in the art as Ziegler-Natta catalysts. Halide residues of these catalyst systems tend to accelerate the premature crosslinking of portions of the ethylene polymer within the extruder, accentuating optical degradation. In fact, optical degradation not only adversely affects the optical and mechanical properties of the extruded film, but in addition, can cause severe problems with respect to maintaining the stability of the "bubble" during the film forming process.
In addition to the problem of optical degradation, low density, ethylene polymers, as previously described, because of their narrow molecular weight distribution and related rheological characteristics, must be processed through relatively wide extruder die gaps, on the order of 60 to 125 mils, to prevent melt fracture, that is, extrudate irregularities, at commercial film extrusion rates. The use of wide extrusion die gaps, however, undesirably increases the amount of orientation in the machine direction which, in turn, promotes directional weakness due to the resultant unbalanced orientation.